In a CDMA mobile communication system, common pilot channels that are transmitted from a base station to mobile stations (CPICH (Common Pilot Channels), P-CCPCH (Primary-Common Control Physical Channels), and S-CCPCH (Secondary-Common Control Physical Channels)) are controlled at a power that enables reception even by mobile stations located at a cell boundary.
FIG. 1 shows the transmitter of a CDMA base station device that relates to the present invention. In FIG. 1, common-channel data 302 that are transmitted from a higher-order station (not shown) are applied as input to common-channel transmitter (common channel 1) 32-1.
In common-channel transmitter 32-1, channel coding unit 321-1 subjects common-channel data 302 to a bit coding process depending on the bit rate and data content to convert common-channel data 302 to a format for wireless transmission. After being converted to the wireless transmission format, common-channel data 302 are applied as input to spreader 322-1. Spreader 322-1 spreads the common-channel data that were supplied from channel coding unit 321-1 by a spreading code whose coding rate accords with the bit rate and supplies the spread data to multiplex/synthesis unit 33. The output of multiplex/synthesis unit 33 is supplied to antenna 36 by way of D/A converter 34 and wireless unit 35.
The transmitter of the CDMA base station device includes a plurality of common-channel transmitters 32-1-32-N for use by CPICH or P-CCPCH channels. Common-channel transmitters 32-2-32-N are of the same configuration as the above-described common-channel transmitter 32-1.
The transmitter of a CDMA base station device further includes a plurality of dedicated-channel transmitters 31-1-31-M. A bit coding process and spreading process that accord with bit rate and data content are carried out for each user in dedicated-channel transmitters 31-1-31-M.
In other words, dedicated-channel data 3011-301M for each user are applied as input to dedicated-channel transmitters 31-1-31-M. For example, in dedicated-channel transmitter 31-1, channel coding unit 311-1 first subjects dedicated-channel data 3011 to a bit coding process that accords with bit rate and data content to convert dedicated-channel data 3011 to a format for wireless transmission. After being converted to the format for wireless transmission, dedicated-channel data 3011 are applied as input to spreader 312-1. Spreader 312-1 spreads the dedicated-channel data supplied from channel coding unit 311-1 by a spreading code whose coding rate accords with the bit rate and supplies the spread data to multiplex/synthesis unit 33.
Although not shown in the figure, dedicated-channel transmitters 31-2-31-M also have the same configuration as dedicated-channel transmitter 31-1 described above and have the same operations as dedicated-channel transmitter 31-1.
Multiplex/synthesis unit 33, based on the power value of each channel, synthesizes the spread data of each of the common channels from common-channel transmitters 32-1-32-N and the spread data of each dedicated channel from dedicated-channel transmitters 31-1-31-M. A designated power value is transmitted from the higher-order station (base station control device) to the CDMA base station device as information indicating the transmission power of each common channel. In the CDMA base station device, the transmission power designated by the higher-order station is applied as input to multiplex/synthesis unit 33. The CDMA base station device decodes the TPC (Transmission Power Control) bits from mobile stations (not shown) to control transmission power for each dedicated channel. A transmission power value that is controlled in this way is applied as input to multiplex/synthesis unit 33.
The higher-order station designates as the transmission power of the common channel a power value that can be received by mobile stations within the range from the vicinity of the base station to the cell boundary. As a result, a mobile station in the vicinity of the base station receives a wireless signal at a power that is greater than necessary for the common channel.
In addition, when a new service such as a MBMS (Multimedia Broadcast Multicast Service) is applied, S-CCPCH is used to execute a Point-to-Multipoint service. In MBMS, a plurality of mobile stations use S-CCPCH to receive the same signal, whereby a service of distributing television images or data can be provided. Because dedicated channels are not used in this case, interference power received by a mobile station from other mobile stations does not change.
However, when a new S-CCPCH dedicated to MBMS is set and the transmission power for the S-CCPCH is set, the total transmission power value of the base station increases, whereby the interference power increases on the mobile station side, and the reception SIR (Signal-to-Interference Ratio) of the dedicated channels decreases. In such a case, mobile stations submit requests to the base station to increase the transmission power of the dedicated channels, whereby the maximum predetermined number of users of dedicated channels in a cell in some cases decreases.
On the other hand, whether mobile stations that wish to use a MBMS are located either in the vicinity of the base station or at the cell boundary, transmission of the S-CCPCH for use in MBMS communication is controlled at the same power regardless of the location of the mobile stations. As a result, when mobile stations that wish to receive MBMS are present only in the vicinity of the base station, the transmission control of the common channel is implemented at a power that is greater than necessary in the cell.
As a technology related to the present invention, a method has been proposed by the inventor of the present application for limiting the transmission power value of common channels that is greater than necessary according to which, when a common channel dedicated for use by a new service such as MBMS has been set, the transmission power of the added common channel is controlled to a low level if mobile stations that receive the service that uses the added common channel are present only in the vicinity of the base station.
FIG. 2 shows the transmitter of a CDMA base station device in the above-described related art. In FIG. 2, transmission power generator 413, taking transmission power value 403 designated from the higher-order station (not shown) as a reference, uses effective paths 421-42m in dedicated-channel receivers 49-1-49-m to detect users at the most remote effective-path positions among the dedicated channels that are using a common channel that has been added.
Dedicated-channel receiver 49-1 includes despreader 491-1 and bit decoder 492-1. Dedicated-channel receivers 49-2-49-m also have the same configuration as dedicated-channel receiver 49-1 and include despreaders 491-2-491-m and bit decoders 492-2-492-m. Each of despreaders 491-1-491-m receives the output of A/D converter 48. The input of A/D converter 48 is connected to antenna 46 by way of wireless unit 47.
Transmission power generator 413 determines whether to alter the transmission power of common channels that are added based on the effective-path positions of users that were detected and supplies the common channel transmission power value following the determination to multiplex/synthesis unit 42. The output of multiplex/synthesis unit 42 is supplied to antenna 45 by way of a D/A converter and wireless unit 44.
According to the above-described related art, the effective-path search positions of dedicated channels that are being used at the time of addition are monitored, and when mobile stations that receive a service that uses an added common channel are present only in the vicinity of the base station, the transmission power of the added common channel is reduced to enable a limitation of common channel power transmission that is greater than necessary.
Transmission power control that is related to the present invention includes the art disclosed in JP-A-2004-215305 (Patent Document 1) and JP-A-2004-201095 (Patent Document 2).